[26.01] Irradiance Variability

J. L. Lean (E. O. Hulburt Center for Space Research, NRL)

From the solar interior emerges magnetic flux that alters
the Sun’s electromagnetic radiation, producing irradiance
variability on a wide range of time scales. The
organization, outward extension and expansion of magnetic
fields structure the solar atmosphere and cause the entire
electromagnetic spectrum to vary in different ways,
depending on wavelength. Surface magnetic fields produce
variations in near UV, visible and near IR spectra which
emerge from the photosphere, and comprise the bulk of total
irradiance. Yet the correlation of daily total irradiance
and total magnetic field strength is poor; rather, the net
effect of two different magnetic features – dark sunspots
and bright faculae – better account for the variations
observed in total irradiance during the solar cycle. The
relationship of UV irradiance with magnetic flux is more
direct since bright active regions control the global
variations in both these quantities. Connecting X-ray and
short wavelength EUV irradiance variations to the solar
interior requires the extrapolation of emerging surface
magnetic fields to the corona and assumptions about the
relationship of their strengths and topologies with coronal
pressure. Knowledge of the emergence, evolution, transport
and decay of magnetic flux is thus a key to understanding
and forecasting solar irradiance variability at all
wavelengths. Identification of sizeable magnetic regions on
the side of the Sun far from Earth may enable EUV and X-ray
irradiance forecasts and subsequent space weather effects on
time scales of days to weeks. On multi-decadal time scales
improved knowledge of the sub surface dynamo and surface
transport processes may help constrain secular solar
irradiance evolution, needed for climate change attribution.
Also needed is quantification of the association between the
closed flux that controls irradiance variability and the
open flux that extends into the heliosphere. Although
utilized frequently to infer irradiance variability, proxies
of long-term solar activity in tree-rings and ice-cores
actually reflect heliospheric modulation of galactic cosmic
rays. Funded by NASA and ONR.